From: Robert Dober <robert.dober@...>
Date: 2007-10-24T05:16:48+09:00
Subject: [ANN] Ruby-Traits 0.2 released (sorry long)

Hi list

after some very interesting input I decided to rewrite Ruby-Traits completely.

http://rubyforge.org/frs/?group_id=4642&release_id=15737

I have decided to implement two different kinds of traits:
* ruby-traits
and
* pure-traits.

In detail:
* ruby-traits are mimicking some of the properties of traits but violate the
flattening property. They implement some of the more dynamic features
of modules and are in fact module based. The code is heavily influenced
by Thomas Sawyer's traits from Facets. Many thanks to Thomas for the
permission to use his code. Note that Thomas' traits of Facets is yet another
different beast.

* pure-traits implement traits in the strict sense of their definition
here: http://portal.acm.org/ft_gateway.cfm?id=1028771&type=pdf&coll=GUIDE&dl=GUIDE&CFID=7912496&CFTOKEN=77115102

They obey the flattening property, using a trait is exactly as defining
its methods inline, no modules are used and inheritance is unchanged.
All trait combination rules are obeyed (associative, communative, e.g.)

The main purpose of pure-traits is to play around with them and to see the
benefits and shortcomings of traits based design.

The syntax has changed, again inspired by Factes, trait aliasing is done
via #* now, thus traits aliased and combined can be expressed more
naturally:
  t1 * { :t1_a => :a } + t2 * { :t2_a => :a }
works now as expected.

In addition to the traditional trait operations +, - and *, I have
implemented & to allow of selective usage of methods.
I believe that the usage of this is normally a bad sign for the design
but sometimes it might be a handy feature.

-------------------------------------------------------------------------------------
               Pure Traits
-------------------------------------------------------------------------------------
# Traits are Composable Units of Behavior. they are described here in detail
# The interested can look here
#  http://portal.acm.org/ft_gateway.cfm?id=1028771&type=pdf&coll=GUIDE&dl=GUIDE&CFID=7912496&CFTOKEN=77115102
#  for a detailed description.
#
# For the Rubyist Traits can be explained in some short phrases quite
effectively:
#
# * Traits are a Collection of Composable Behavior (e.g. methods)
#   They ressemble a lot to modules.
    t1 =  trait{ def a; 42 end }
    assert_kind_of Trait, t1
    t2 = Trait::new { def b; 46 end }
# * Traits can be composed by operations as + (union), &
(intersection), * (aliasing)
#   and - (substraction). The result of such a trait composition is
always a new trait.
#   As a matter of fact traits are immutable( with the exception of
metaprogramming techniques ).
    t3 = t1 + t2
    assert_kind_of Trait, t3
    c1 = Class::new {
      use t3
    }
    assert_equal 42, c1.new.a
    assert_equal 46, c1.new.b
# * When a trait is composed *and only in that case* conflicts might
occur. Conflicts
#   can only occur during trait unification (that is +) in case the two unified
#   traits contain methods with the same name, these methods are
replaced by conflict
#   raising methods. It is the responsability of the using entity
(Module, Trait or Class)
#   to resolve the conflict.
    t4 = Trait::new do
      def a; :t4 end
    end
    t5 = Trait::new do
      def a; :t5  end
    end
    c1 = Class::new do
      use t4, t5 ### this is exactly the same as "use t4 + t5"
    end
    c2 = Class::new do
      use t4
      use t5 ### This is *not* the same as the above
    end
    assert_raise TraitsConflict do
      c1.new.a
    end
    assert_equal :t5, c2.new.a
# The second case is distinct to the first in that a "use" of a trait is a
# transparent operation in that the trait's methods are just defined as if they
# were defined in the using entity (c2 in our example above).
# Almost as if they were macroexpanded
#
# As mentioned above it is up to the using entity to resolve such
conflicts, sometime
# such conflict resolution needs access to the conflicting methods, in
these cases
# aliasing is our friend.
    t6 = trait do
      def the_answer; 22 end
    end
    t7 = trait do
      def the_answer; 20 end
    end
    c3 = Class::new {
      use t6 * {:first_answer => :the_answer} +
          t7 * {:second_answer=> :the_answer}
      def the_answer; first_answer + second_answer end
    }
    assert_equal 42, c3.new.the_answer
# Sometimes of course we do not want to resolve the conflict
explicitly but we just
# want to avoid it by choising from which trait we want to use the method, again
# trait composition shall be used to achieve that task.
    m1 = Module::new do
      use t6 + ( t7 - :the_answer )
    end
    assert_equal 22, Class::new{ extend m1 }.the_answer
# * When traits are used they obey to the Flattening Property, that is
the using entity
#   does not inherit from the trait but just get its methods defined.
#   Again one can very nicely understand what is going on when
thinking of macro expansion.
    t= trait {
      def a; 1 end
    }
    c = Class::new {
      use t
    }
    o = c.new
    assert_equal 1, o.a
    t.send :add_method, :a do 2 end
    assert_equal 1, o.a
    assert_equal 1, c.new.a
    assert_equal 2, t.to_class.new.a
# * Calls to super in trait methods are dynamically resolved at
runtime, as is the case
#   for modules.
    t8 = trait { def a
     super * 7  end }
    c4 = Class::new { def a; 3 end }
    c5 = Class::new( c4 ) {
      use t8
    }
    assert_equal 21, c5.new.a
# Again the flattening property can be tested with the semantics of
super, the usage of the trait
# just defines methods, they can be replaced with methods in the class
later, but they are not
# accessible via super, as they were if they had been included via modules.
    c5.module_eval do
      def a; super.succ end
    end
    assert_equal 4, c5.new.a

-------------------------------------------------------------------------------------
               Ruby Traits
-------------------------------------------------------------------------------------
# Traits are Composable Units of Behavior. they are described here in detail
# The interested can look here
#  http://portal.acm.org/ft_gateway.cfm?id=1028771&type=pdf&coll=GUIDE&dl=GUIDE&CFID=7912496&CFTOKEN=77115102
#  for a detailed description.
#
# For the Rubyist Traits can be explained in some short phrases quite
effectively:
#
# * Traits are a Collection of Composable Behavior (e.g. methods)
#   The ressemble a lot to modules.
    t1 =  trait{ def a; 42 end }
    assert_kind_of Trait, t1
    t2 = Trait::new { def b; 46 end }
# * Traits can be composed by operations as + (union), &
(intersection), * (aliasing)
#   and - (substraction). The result of such a trait composition is
always a new trait.
#   As a matter of fact traits are immutable( with the exception of
metaprogramming techniques ).
    t3 = t1 + t2
    assert_kind_of Trait, t3
    c1 = Class::new {
      use t3
    }
    assert_equal 42, c1.new.a
    assert_equal 46, c1.new.b
# * When a trait is composed *and only in that case* conflicts might
occur. Conflicts
#   can only occur during trait unification (that is +) if and only if
the two unified
#   traits contain methods with the same name, these methods are
replaced by conflict
#   raising methods. It is the responsability of the using entity
(Module, Trait or Class)
#   to resolve the conflict.
    t4 = Trait::new do
      def a; :t4 end
    end
    t5 = Trait::new do
      def a; :t5  end
    end
    c1 = Class::new do
      use t4, t5 ### this is exactly the same as "use t4 + t5"
    end
    c2 = Class::new do
      use t4
      use t5 ### This is *not* the same as the above
    end
    assert_raise TraitsConflict do
      c1.new.a
    end
    assert_equal :t5, c2.new.a
# The second case is distinct to the first in that a "use" of a trait is a
# transparent operation in which the trait's methods are just defined as if they
# were defined in the using entity (c2 in our example above).
#
# As mentioned above it is up to the using entity to resolve such
conflicts, sometime
# such conflict resolution needs access to the conflicting methods, in
these cases
# aliasing is our friend.
    t6 = trait do
      def the_answer; 22 end
    end
    t7 = trait do
      def the_answer; 20 end
    end
    c3 = Class::new {
      use t6 * {:the_answer=>:first_answer} +
          t7 * {:the_answer=>:second_answer}
      def the_answer; first_answer + second_answer end
    }
    assert_equal 42, c3.new.the_answer
# Sometimes of course we do not want to resolve the conflict
explicitly but we just
# want to avoid it by choising from which trait we want to use the method, again
# trait composition shall be used to achieve that task.
    m1 = Module::new do
      use t6 + ( t7 - :the_answer )
      def the_answer; super.to_s.to_i(20) end
    end
    assert_equal 42, Class::new{ extend m1 }.the_answer
# * When traits are used they obey to the Flattening Property, that is
the using entity
#   does not inherit from the trait but just get its methods defined
exactly as if the defs
#   of the trait were macro expanded into the using entity.
#   This however is something I have not implemented in ruby-traits
(only in pure-traits).
#   As one can notice in the super example above the method
t6#the_answer can be accessed via
#   super in the method m1#the_answer
#   There are two reasons for this, (a) I wanted to use Thomas'
elegant and simple approach
#   and (b) this is a behaviour Rubyists are rather fond of, so I
really thaught that
#   it might be a good idea to do this. The price to pay is that
traits still behave a little
#   bit too much as Modules, the following example shows the down side
of this approach
    t= trait {
      def a; 1 end
    }
    c = Class::new {
      use t
    }
    assert c < t
    o = c.new
    assert_equal 1, o.a
    t.send :add_method, :a do 2 end
    assert_equal 2, o.a
# Another downside of this is that Trait#define_method,
Trait#remove_method and friends
# cannot be undefined, using them on traits directly would however
endanger correct
# conflict detection or trigger false conflicts!
# In order to do correct traits metaprogramming Traits#add_method and
Traits#del_method,
# both are private, shall be used.
# * Calls to super in trait methods are dynamically resolved at
runtime, as is the case
#   for modules.
#   This even works together with the usage of super as shown above
    t8 = trait { def a; super * 7  end }
    c4 = Class::new { def a; 3 end }
    c5 = Class::new( c4 ) {
      use t8
      def a; super * 2  end
    }
    assert_equal 42, c5.new.a
#
    # Trait#to_class simply is a convenience method that
    # simply creates an anonymous class from this trait.
"t.to_class(parent=Object)" is short for
    # "Class::new(parent)"{ use t }
    t9 = trait { def a; :a9 end; def b; :b9 end }
    t10 = trait { def b; :b10 end; def c; :c10 end }
    c6 = ( t9 + t10 ).to_class
    assert_equal :a9, c6.new.a
    assert_equal :c10, c6.new.c
    assert_raise TraitsConflict do c6.new.b end

-------------------------------------------------------------------------------------------------------
Robert